Originally Posted by hasenwerk
I think it is all what you are looking for. For the front, if I had to buy new I would go 21" - just because of the better off road abilities and the fact that a 90/90 21" is much cheaper than a 19" front - the old R100 / R80 GS had 21" as does the F800 - so that speaks some merit of the 21" - tubeless can most certainly be done using a R100 rim but they are not nearly as strong as the Excel rim.
The 17" rear is the way to go - I considered the 18" for my 1100 GS and my 30" inseam and the good choice of 17" rubber made me stay with 17"
All that being said I have a Woody's 21" front and an OE 17" rear on my bike - 90/90 21 on the front and 140/80 17 on the rear - tubes on both ends. I am happy with the whole setup!
90/90-21 on the front gives you better off road abilities but at the cost of worse on road handling due to higher trail and smaller contact patch up front due to loss of 30mm width with a 90mm tread.